It is known to provide a viewing surface that is optically graded to vary in its transmissive or reflective capacity along a longitudinal or circumferential path as part of a position sensor. Sensors passing along such path are exposed to varying illumination, corresponding to the location of such sensors along the path.
A paper authored by V. D. Brown and W. S. Newman and published by AT&T in the AT&T Technical Digest No. 78 of Jul. 1986 (page 5) shows a "V" shaped light transmitting pattern formed in a mask that is wrapped circumferentially around the outer surface of a transparent cylinder source containing a light source in its core. Rotation of the "V" shaped pattern past a light sensor provides a sensor output that corresponds to the rotational displacement of the cylinder.
This same publication depicts a tapering lighttransmitting region formed in a mask carried on the face of a transparent rotatable disc or wheel. Light transmitted through the disc to a sensor provides a sensor output which is proportional to the rotational displacement of the wheel.
In U.S. Pat. No. 5,666,236 of Bracken et al issued to Iomega Corporation for a computer disc drive the position of a read-head arm is determined by sensing a "gray-scale" pattern, which includes an array of parallel gradually thinning lines, based on the intensity of light reflected from the pattern as it passes before a sensor.
Both of these prior art references rely upon obtaining a light signal from a single optical source using a single light modulating pattern and a single light sensor.
Another reference, U.S. Pat. No. 5,153,472 to Karidis et al depicts an actuator for positioning a probe which includes as a position sensing device two oppositely oriented, parallel, tapered windows (FIGS. 6, 6A; ref. 76, 77) mounted in a single plate. Light, optionally from a common source shone through these triangular windows is intercepted by two independent light sensors. By reason of the reversed orientation of the two windows, the intensity of light received by the respective sensor varies in a complementary fashion. Without stating how the output signals of the light sensors are processed, this reference observes that this variable light limiting plate can be used to determine its precise position.
While the prior art examples correlate position with the intensity of light modulated by an optical pattern, the full potential of such arrangements has not been recognized or exploited.
Absent from these references is any suggestion that the use of dual complementary images can serve to reduce the noise and errors inherent in sensors of this type. In particular, there is no suggestion as to the advantageous ways in which such dual outputs may be combined to produce measurement of improved fidelity and precision. This invention addresses further improvements in the utilization of this type of effect.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.